Research on the detection method of microcracks in aluminum thin plates based on nonlinear ultrasonic theory

Nonlinear ultrasonic technology has demonstrated high sensitivity in the early-stage detection of microcracks and defects. This paper explores the feasibility of applying nonlinear ultrasonic testing technology to the detection of microcracks in aircraft metal skins through numerical simulation. An aircraft skin model with ultrasonic transducers placed on its surface is used to measure the second harmonic nonlinear elastic response related to material microcracks. Then, multiple sets of piezoelectric wafers are used to collect time-domain signals, which are converted into frequency-domain signals through fast Fourier transform to obtain the frequency changes of the signals. The microcracks in the material are detected by quantifying the changes in the frequency-domain signals. The numerical experiment results show that using the normalized second-order nonlinear parameter can obtain relatively accurate damage information. Unlike traditional linear ultrasonic technology, the nonlinear ultrasonic testing method can detect material damage on aluminum alloy thin plates at the initial stage of crack formation when it is invisible to the naked eye.